JP2015010248A - Two-phase stainless steel for urea plant - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a two-phase stainless steel for urea plant having excellent intergranular corrosion resistance under 65 mass% boiling nitric acid environment and also having excellent hot workability.SOLUTION: A two-phase stainless steel has a chemical composition comprising C: 0.05 mass% or less, Si: 1 mass% or less, Mn: 0.3-2 mass%, P: 0.04 mass% or less, S: 0.002 mass% or less, Al: 0.1 mass% or less, N: 0.05-0.4 mass%, Ni: 5-8 mass%, Cr: 20-27 mass%, Mo: 2-5 mass%, B:≤0.01 mass%, and Ca: more than 0.0005 mass% and 0.01 mass% or less, the P and B satisfying (mass%P×mass%B)≤12×10, with the balance being Fe and inevitable impurities.

Description

  The present invention relates to a duplex stainless steel for a urea plant, and specifically to a duplex stainless steel for a urea plant that has excellent intergranular corrosion resistance and excellent hot workability. In addition, the said duplex stainless steel in this invention is not limited to a thin steel plate, Any of a thick steel plate, a shape steel, a bar, a wire, a steel pipe, etc. may be sufficient.

  Among stainless steels, duplex stainless steel is an excellent material that combines strength and corrosion resistance. It is used in high chloride environments such as seawater, and in severe corrosive environments such as oil wells and fertilizer plants. Is widely used. One of the above uses is a reactor or a heat exchanger of a plant that produces urea as a supply source of nitrogen, which is a component of fertilizer.

  In the urea production process, it is known that a highly corrosive intermediate (ammonium carbamate) is generated in the process of synthesizing urea, causing intergranular corrosion. Therefore, the above-mentioned plant has conventionally been excellent in intergranular corrosion resistance, for example, UNS No. S32906, UNS No. A high grade duplex stainless steel having excellent intergranular corrosion resistance such as S32808 has been used. This is because when a graded duplex stainless steel is used, intergranular corrosion occurs, and once intergranular corrosion occurs, the corrosion part at the intergranular region is the starting point, and stress corrosion cracking (SCC) This may cause catastrophic damage to the urea plant.

The duplex stainless steel used in the urea plant is an intergranular corrosion test (so-called “Huey test”) using a 65 mass% boiling nitric acid solution defined in ASTM A262 Practice C. In many cases, it is determined whether or not to adapt to a urea plant depending on whether (corrosion rate) satisfies 0.2 g / m ² · hr or less.

  The potential range of 65 mass% boiling nitric acid used in the above intergranular corrosion test extends from the passive state region to the hyperpassive region. The cause of the intergranular corrosion is the preferential corrosion of the Cr-deficient layer due to Cr carbide precipitation, It is considered to dissolve Ni phosphide precipitated at the boundary, and to dissolve intermetallic compounds such as σ phase, χ phase, and Laves phase.

  As a high corrosion resistance stainless steel excellent in intergranular corrosion resistance, for example, Patent Document 1 discloses a grain boundary of the Laves phase and the χ phase by controlled cooling or reheating in a specific temperature range of a cooling process after solution treatment. Stainless steel with suppressed precipitation is also disclosed in Patent Document 2 by reducing the Si concentration and the P concentration, thereby suppressing the precipitation of χ phase and P compound on the grain boundary and increasing the intergranular corrosion resistance. Stainless steel is disclosed. Furthermore, Patent Document 3 discloses that the content (mass%) of C, N, Mn, and Ni is 1.5 × Ni + Mn + 65 × (C + N) −5 × Si−2.5 ≦ 52−2.3 × (Ni + Mn ) −200 × (C + N), etc., B is 5 mass ppm or less, and one or more selected from Ti, Nb, V, Hf and Ta are 1.0 mass in total. Stainless steel having excellent nitric acid corrosion resistance after cold working is disclosed.

  However, all of the above stainless steels are austenitic, and these technologies are structural materials used in high-temperature high-pressure water environments such as nuclear reactors or high-concentration nitric acid environments such as nuclear fuel reprocessing facilities. It is targeted. The corrosive environment for the above application is much more severe than the 65 mass% boiling nitric acid corrosion test in the Huey test described above. However, the austenitic stainless steel needs to be thickened because of its low strength, and is expensive because of its high Ni content, and is not suitable for use in a urea plant.

  Therefore, in place of austenitic stainless steel, use of duplex stainless steel having high strength and capable of being thinned and inexpensive is being studied. As a duplex stainless steel having excellent corrosion resistance, for example, Patent Document 4 specifies the amount of Si to increase the solid solubility of N, and suppresses precipitation of chromium nitride at grain boundaries during air cooling after solutionization. Thus, a duplex stainless steel is disclosed in which intergranular corrosion due to sensitization is prevented. Patent Document 5 discloses a duplex stainless steel in which the pitting corrosion resistance is improved by reducing the Ca concentration in the steel and suppressing CaO-containing inclusions.

Japanese Patent Laid-Open No. 05-271752 Japanese Patent Laid-Open No. 04-36440 Japanese Patent Laid-Open No. 08-013095 JP 2006-233308 A JP 2009-007638 A

However, the technique disclosed in Patent Document 4 is a so-called intergranular corrosion resistance of steel, which is far less corrosive environment (corrosion potential) than the above “huey test”, and uses a sulfuric acid / copper sulfate solution as a test solution. Evaluated by “Strauss test”. Therefore, whether the steel disclosed in Patent Document 4 is applicable to a urea plant is unknown. The technique disclosed in Patent Document 5 is intended to improve the pitting corrosion resistance, and no investigation has been made on the intergranular corrosion resistance.
As described above, in the prior art, there has been no investigation of intergranular corrosion resistance of duplex stainless steel in a 65 mass% boiling nitric acid environment.

  The present invention has been made in view of the above-mentioned problems in the prior art, and an object thereof is to provide a duplex stainless steel for urea plants having excellent intergranular corrosion resistance in a 65 mass% boiling nitric acid environment. It is in.

  The inventors have intensively studied to solve the above problems. As a result, by lowering B, segregation of B at the grain boundary and precipitation of B compound at the grain boundary are prevented to suppress a decrease in grain boundary binding energy, and by lowering P, the grain boundary of the P compound is obtained. It is possible to obtain a duplex stainless steel having excellent intergranular corrosion resistance even in a 65 mass% boiling nitric acid environment by reducing Si that promotes the formation of P compounds at the same time as suppressing the precipitation of P Lowering B may cause a decrease in hot workability due to S, and in order to avoid such problems, the inventors have found that the addition of Ca is effective and led to the development of the present invention. It was.

That is, the present invention includes C: 0.05 mass% or less, Si: 1 mass% or less, Mn: 0.3 to 2 mass%, P: 0.04 mass% or less, S: 0.002 mass% or less, Al: 0.1 mass %: N: 0.05 to 0.4 mass%, Ni: 5 to 8 mass%, Cr: 20 to 27 mass%, Mo: 2 to 5 mass%, B: ≦ 0.01 mass%, Ca: more than 0.0005 mass% A component composition containing 0.01 mass% or less, and P and B satisfying (mass% P × mass% B) ≦ 12 × 10 ⁻⁵ , with the balance being Fe and inevitable impurities It is a duplex stainless steel for urea plants characterized by having.

In the duplex stainless steel for urea plant of the present invention, the average corrosion rate of the non-sensitized specimen in the intergranular corrosion test using the 65 mass% boiling nitric acid solution defined in ASTM A262 Practice C is 0.2 g / m ² · It is less than hr.

Further, the duplex stainless steel for urea plant of the present invention is characterized in that the Si, P and B satisfy (mass% Si × mass% P × mass% B) ≦ 5 × 10 ^−5. .

Moreover, the duplex stainless steel for urea plants of the present invention is characterized in that the Si and Al satisfy (mass% Si × mass% Al) ≧ 1 × 10 ⁻³ .

  Moreover, the duplex stainless steel for urea plants of this invention is characterized by containing Mg: 0.0001-0.01mass% further in addition to the said component composition.

  According to the present invention, duplex stainless steel having excellent intergranular corrosion resistance in a 65 mass% boiling nitric acid environment and excellent hot workability can be provided at low cost. It can be suitably used as an exchanger.

It is a graph which shows the influence which (mass% Pxmass% B) has on intergranular corrosion resistance. It is a graph which shows the influence which (mass% Sixmass% Pxmass% B) has on intergranular corrosion resistance.

Inventors are known to segregate or precipitate at grain boundaries in order to improve the intergranular corrosion resistance of duplex stainless steels in a 65% boiling nitric acid environment, and the above-mentioned Paying attention to the influence of Si that promotes the precipitation of P, the following experiment was conducted.
<Experiment 1>
Contains Cr: 25 mass%, Ni: 6 mass%, Mo: 3 mass%, N: 0.17 mass%, and further contains Si, P, S, Al, N, and B in the composition shown in Table 1, and the balance The steel made of Fe is melted in the atmosphere in a magnesia crucible using a high-frequency induction furnace to form a CaO—SiO ₂ —Al ₂ O ₃ —MgO—F slag, desulfurized, and then cast into a mold to give 20 kg. It was a steel ingot.
Next, the ingot is hot-rolled, annealed, pickled, and further cold-rolled to obtain a cold-rolled sheet having a thickness of 5.0 mm, and then annealed and pickled to obtain a cold-rolled annealed sheet. Thereafter, a corrosion test piece having a width of 20 mm × a length of 25 mm × a thickness of 4 mm was taken from the cold-rolled annealed plate.

Subsequently, the above-mentioned corrosion test piece was used for a grain boundary corrosion test using a 65% boiling nitric acid solution defined in ASTM A262 Practice C. In the above corrosion test, 5 batch immersion tests were performed with 48 hours of 1 batch while renewing the 65% nitric acid corrosive solution, corrosion weight loss was measured, the corrosion rate was calculated, and the intergranular corrosion resistance was evaluated. . The corrosion test piece was not subjected to sensitizing heat treatment.
The evaluation of the intergranular corrosion resistance is as follows. If the average corrosion rate of 5 batches is 0.20 g / m ² · hr or less, it can be determined that it can be applied to a urea plant, so 0.20 g / m ² · hr The intergranular corrosion resistance is inferior (×), the intergranular corrosion resistance is 0.20 g / m ² · hr or less (◯), and the 0.19 g / m ² · hr or less is the intergranular corrosion resistance. The corrosivity was judged as excellent (（).

The test results are shown in Table 1. FIG. 1 shows the influence of the product of P and B contents (mass% P × mass% B) on the intergranular corrosion rate. From FIG. 1, when (mass% P × mass% B) exceeds 12 × 10 ⁻⁵ , the intergranular corrosion rate also exceeds 0.20 g / m ² · hr, in other words, use in a urea plant. It can be seen that (mass% P × mass% B) needs to be 12 × 10 ⁻⁵ or less in order to make the intergranular corrosion rate, which is a criterion, 0.20 g / m ² · hr or less.

FIG. 2 shows the influence of the product of the contents of Si, P and B (mass% Si × mass% P × mass% B) on the intergranular corrosion rate. From this figure, it is necessary to set (mass% Si × mass% P × mass% B) to 5 × 10 ⁻⁵ or less in order to make the intergranular corrosion rate 0.20 g / m ² · hr or less. I understand.

Further, as can be seen from Table 1, (mass% P × mass% B) is 12 × 10 ⁻⁵ or less, and (mass% Si × mass% P × mass% B) is 5 × 10 ⁻⁵ or less. In this case, the intergranular corrosion rate can be reduced to 0.19 g / m ² · hr or less.

From the above results, in order to use the Fe-25Cr-6Ni-3Mo-0.17N alloy in the urea plant, (mass% P × mass% B) is set to 12 × 10 ⁻⁵ or less, or ( mass% Si × mass% P × mass% B) is required to be 5 × 10 ⁻⁵ or less.

<Experiment 2>
From the results of <Experiment 1>, it is effective to reduce B for use in a urea plant. However, B is an element that has been positively added conventionally as an element that suppresses hot brittleness caused by S and improves hot workability, and the reduction of B causes a decrease in hot workability. Is expected. Therefore, in order to evaluate workability at high temperatures, as shown in Table 2, a steel having the same composition as Table 1 of <Experiment 1> was melted except that the B content was 0.0001 mass%. A 20 kg ingot was manufactured, and the influence of S and deoxidizer (Al, Si) affecting desulfurization on hot workability was investigated.

  A round bar tensile test piece of diameter: 8 mmφ x length: 70 mm was sampled from an ingot having the composition shown in Table 2 by machining, and a high temperature tensile test was performed using a hot working reproduction test device (Cermec Master Z). It was used for. In the tensile test, the round bar tensile test piece was heated to 1170 ° C. and held for 90 seconds, cooled to 900 ° C. at 10 ° C./s, held at that temperature for 30 seconds, and then a high strain rate of 100 mm / s. Tensile test was performed at, and the drawing ratio RA was determined from the fracture surface after the test. In addition, the evaluation of hot workability is based on the conventional experience that, when the drawing ratio RA is 60% or more, ear cracks during hot rolling do not occur. Those having an RA of less than 60% were judged to be inferior in workability “x”.

The experimental results are also shown in Table 2. From Table 2, the component composition that provides good hot workability (RA ≧ 60%) has a product of Si and Al content (mass% Si × mass% Al) of 1 × 10 ⁻³ or more and S : It turned out that it is the range of 0.002 mass% or less.
The present invention has been developed by further studying the above findings.

Next, the component composition that the duplex stainless steel of the present invention should have will be described.
C: 0.05 mass% or less C is an austenite stabilizing element. However, if added in a large amount, it combines with Cr, Mo and the like to form carbides, lowers the amount of solid solution Cr and solid solution Mo in the base material, and lowers corrosion resistance. Therefore, C is limited to 0.05 mass% or less. Preferably it is 0.03 mass% or less.

Si: 1 mass% or less Since Si is an element that promotes precipitation of P compounds at grain boundaries and increases the intergranular corrosion sensitivity, the upper limit is set to 1 mass%. However, since Si is also a deoxidizing element, addition of 0.05 mass% or more is preferable. More preferably, it is the range of 0.1-0.8 mass%.

Mn: 0.3-2 mass%
Mn is an element having a deoxidizing action. Moreover, since it is also an austenite formation element, in order to control the phase ratio of an austenite and a ferrite, addition of 0.3 mass% or more is required. However, addition exceeding 2 mass% promotes embrittlement by forming a σ phase or a χ phase and lowers the corrosion resistance. Therefore, Mn is set to a range of 0.3 to 2 mass%. Preferably it is the range of 0.5-1 mass%.

P: 0.04 mass% or less P is an element that is inevitably mixed in steel as an impurity, and precipitates at a grain boundary as a P compound, which harms intergranular corrosion resistance and hot workability. It is desirable to reduce as much as possible. Therefore, in the present invention, P is limited to 0.04 mass% or less. Preferably it is 0.03 mass% or less.

S: 0.002 mass% or less S is an element harmful to hot workability, and it is necessary to reduce it to 0.002 mass% or less in order to prevent ear cracks during hot working. More preferably, it is 0.0015 mass% or less, More preferably, it is 0.001 mass% or less.

Al: 0.1 mass% or less Since Al is an effective deoxidizing element, it is preferable to contain 0.001 mass%. Al also has the effect of promoting desulfurization by deoxidation to reduce S and improve hot workability. However, if Al exceeds 0.1 mass%, not only the deoxidation effect is saturated, but also the precipitation of intermetallic compounds is promoted, and the Ca concentration in molten steel is increased to promote the formation of CaO-containing inclusions. In addition, the corrosion resistance is reduced. Therefore, Al is limited to 0.1 mass% or less. A preferable range of Al is 0.001 to 0.05 mass%.

N: 0.05 to 0.4 mass%
N is a strong austenite-forming element, and, like Cr and Mo described later, is an element effective for improving corrosion resistance and suppressing precipitation of intermetallic compounds. Therefore, N is contained in an amount of 0.05 mass% or more. On the other hand, when the content exceeds 0.4 mass%, the hot deformation resistance is increased to inhibit the hot workability, and it is difficult to maintain the two-phase structure. Therefore, N is set to a range of 0.05 to 0.4 mass%. Preferably it is the range of 0.1-0.3 mass%.

Ni: 5-8 mass%
Ni is an austenite-forming element and is an essential element for maintaining a two-phase structure of austenite and ferrite. If it is less than 5 mass%, it becomes difficult to maintain a two-phase structure. On the other hand, if it exceeds 8 mass%, the austenite structure becomes excessive and becomes an acceleration factor for transpassive corrosion, so that the corrosion resistance decreases. Is in the range of 5-8 mass%. Preferably it is the range of 6-7 mass%.

Cr: 20-27 mass%
Cr is an element that improves the corrosion resistance. In order to obtain the effect, it is necessary to contain 20 mass% or more. However, when added in excess of 27 mass%, formation of intermetallic compounds such as σ phase and χ phase is promoted, and on the contrary, corrosion resistance is lowered. Moreover, Cr is a ferrite-forming element, and excessive addition makes it difficult to maintain a two-phase structure. Therefore, Cr is set to a range of 20 to 27 mass%. Preferably it is the range of 22-26 mass%.

Mo: 2-5 mass%
Mo is an element effective for improving the corrosion resistance against general corrosion and pitting corrosion, and therefore needs to be contained in an amount of 2 mass% or more. However, excessive addition of Mo promotes the formation of intermetallic compounds such as σ phase and χ phase, and reduces intergranular corrosion resistance. Therefore, Mo is set to a range of 2 to 5 mass%. Preferably it is the range of 2.5-3.5 mass%.

B: 0.01 mass% or less B is an element that is extremely effective in improving hot workability. However, when B is added in excess of 0.01 mass%%, B segregates at the grain boundary or the B compound precipitates at the grain boundary, thereby reducing the intergranular corrosion resistance. Therefore, B is limited to 0.01 mass% or less. Preferably it is 0.007 mass% or less.

Ca: more than 0.0005 mass% and less than 0.01 mass% Ca is an element effective for improving hot workability by forming CaS by combining with S, which is harmful to hot workability. In order to acquire an effect, it is necessary to contain more than 0.0005 mass%. However, addition exceeding 0.01 mass% forms inclusions containing CaO, and decreases the corrosion resistance. Therefore, Ca is added in the range of more than 0.0005 mass% and less than 0.01 mass%. Preferably it is the range of 0.001-0.005 mass%.

Ca may be added by a Ca alloy. However, since the yield may not be stable due to volatility, the reaction formula (1) below is reduced with Al added with CaO in slag. It is preferable to supply Ca into the molten steel.
3 (CaO) + 2Al → 3Ca + (Al ₂ O ₃ ) (1)
The parentheses in the above formula indicate the components in the slag, and the underlined portions indicate the components in the molten steel.
Since the chemical reaction formula is an equilibrium reaction, the yield is stabilized. Moreover, Ca added in the said molten steel couple | bonds with S in molten steel, forms CaS by the following (2) formula, and removes S.
Ca + S → (CaS) (2)

In addition to satisfying the above component composition, the duplex stainless steel of the present invention further has P and B represented by the following formulas:
(Mass% P × mass% B) ≦ 12 × 10 ⁻⁵ (3)
It is necessary to satisfy and contain.
P and B are both elements that reduce intergranular corrosion resistance. In the intergranular corrosion test using a 65% boiling nitric acid solution defined in ASTM A262 Practice C, the average corrosion rate of the non-sensitized specimen is This is because, in order to satisfy 0.2 g / m ² · hr or less, as shown in FIG. 1, (mass% P × mass% B) needs to be 12 × 10 ⁻⁵ or less.

Furthermore, in addition to satisfying the above component composition and the formula (3), the duplex stainless steel of the present invention further includes Si, P and B in the following formula (4):
(Mass% Si × mass% P × mass% B) ≦ 5 × 10 ⁻⁵ (4)
It is preferable to contain and satisfy.
As described above, since Si is an element that promotes precipitation of P compounds at grain boundaries and reduces intergranular corrosion resistance, it is preferable to limit Si in addition to P and B. In order to satisfy the average corrosion rate of the test specimens of 0.2 g / m ² · hr or less, as shown in FIG. 2, (mass% Si × mass% P × mass% B) is 5 × 10 ^−5. This is because the following is preferable. In addition, when satisfy | filling the said (3) Formula and (4) Formula simultaneously, it is possible to reduce the average corrosion rate of a non-sensitized test piece to 0.19 g / m < ² > * hr or less.

By the way, B is an element effective for improving the hot workability as described above, and reducing B to satisfy the above expressions (3) and (4) There is a risk of lowering. Therefore, even in such a case, the duplex stainless steel of the present invention has Si and Al represented by the following formula (5) based on Table 2 in order to ensure hot workability:
(Mass% Si ×% Al) ≧ 1 × 10 ⁻³ (5)
It is preferable to contain so that it may satisfy | fill.

The added Al and Si both react with O in the steel to form Al ₂ O ₃ and SiO ₂ to deoxidize the steel, and at the same time, CaO—SiO ₂ —Al ₂ O ₃ —MgO—F. It has the effect of forming hot slag to promote desulfurization and improving hot workability. The mechanism is that Ca is supplied into the molten steel by the reaction of the above-described formula (1) and the following formula (6), and the Ca is combined with S according to the formula (2) to form CaS. is there.
2 (CaO) + Si → 2Ca + (SiO ₂ ) (6)
Since the above reaction between Si and Al occurs independently, the above effect can be synergistically enhanced by the combined addition. Therefore, in the present invention, it is preferable to control their product, that is, (mass% Si × mass% Al) to 1 × 10 ⁻³ or more. More preferably, it is 3 × 10 ⁻³ or more.

Moreover, in addition to the said component composition, 0.01 duplex% or less of Mg can be added to the duplex stainless steel of this invention.
Similar to Ca, Mg is an element effective for improving hot workability, and the above effect can be obtained by adding 0.0001 mass% or more. However, if added over 0.01 mass%, bubble defects due to Mg will occur in the steel, so the upper limit is preferably set to 0.01 mass%.

Next, the manufacturing method of the duplex stainless steel of this invention is demonstrated.
In the duplex stainless steel of the present invention, raw materials such as iron scrap, stainless scrap, ferronickel and ferrochrome are melted in an electric furnace, and degassed by blowing a mixed gas of oxygen and rare gas in an AOD furnace or a VOD furnace. and charcoal refining, burnt lime, Fe-Si alloy, after the addition of Al or the like reduction treatment of Cr oxides in the slag, the addition of fluorite _{CaO-SiO 2 -Al 2 O 3} -MgO-F slag And then deoxidizing and desulfurizing, and further adding Ca and Mg, and then forming a steel slab by a continuous casting method or ingot-bundling rolling method, and then hot rolling the steel slab, or Furthermore, it is preferable to cold-roll and to make various steel materials, such as a thin steel plate, a thick steel plate, a shape steel, a bar steel, and a wire rod.

  Raw materials prepared by adjusting iron scrap, ferrochrome, ferronickel, stainless steel scrap, etc. to a specified ratio are melted in an electric furnace and secondarily refined in an AOD (Argon Oxygen Decarburization) furnace or VOD (Vacuum Oxygen Decarburization) furnace. After adjusting to the main component composition shown in Table 3, it was continuously cast into a steel slab. In addition, the composition of C and S shown in Table 3 is a carbon / sulfur simultaneous analyzer (combustion in an oxygen stream-infrared absorption method), and the composition of N is an oxygen / nitrogen simultaneous analyzer (inert gas). -Impulse heating melting method), and the composition other than the above is a value analyzed using fluorescent X-ray analysis.

  Next, the slab was hot-rolled, and cold rolling and heat treatment were repeated to obtain a cold-rolled coil having a thickness of 5 to 6 mm. At this time, both width end portions of the steel sheet after hot rolling were visually observed over the entire length of the coil, and a coil in which cracks having a length of 10 mm or more occurred at 10 or more locations per 100 m had poor hot workability. (X), 9 or less coils were judged to have good hot workability (◯), and coils with no cracking were judged to have excellent hot workability (◎).

Next, a corrosion test piece having a width of 20 mm, a length of 25 mm, and a plate thickness of 5 mm is taken from the cold rolled coil, and the corrosion solution is renewed by using a 65% nitric acid boiling solution as the corrosion solution without performing sensitizing heat treatment. Then, 5 immersion tests were performed with 48 batches as 1 batch, the corrosion weight loss was measured, the corrosion rate was determined, and the intergranular corrosion resistance was evaluated.
The evaluation of intergranular corrosion resistance, 0.20g ^{/ m} 2 · hr exceed intergranular corrosion resistance is poor (×), the good intergranular corrosion resistance below 0.20g ^{/ m} 2 · hr ( ○) Furthermore, 0.19 g / m ² · hr or less was judged to have excellent intergranular corrosion resistance (◎).

The evaluation results of the hot workability and intergranular corrosion resistance are also shown in Table 3.
No. shown in Table 3. Steel plates 1 to 13 are invention examples that satisfy the conditions of the present invention, and have excellent intergranular corrosion resistance and hot workability. Among them, No. 1 satisfying the condition that (mass% Si × mass% P × mass% B) is 5 × 10 ⁻⁵ or less. Nos. 1 to 11 have a corrosion rate in a 65% nitric acid boiling solution of 0.19 g / m ² · hr or less, and the intergranular corrosion resistance is remarkably excellent.
In addition, No. In No. 3, since (mass% Si × mass% Al) was less than 3 × 10 ⁻³ , S was slightly high at 0.0012 mass%, and it was within the acceptable range, but ear cracking occurred during hot rolling.

On the other hand, any one of Si, P and B is outside the scope of the present invention, and / or (mass% P × mass% B) is 12 × 10 ⁻⁵ or less. 14 to 21 steel plates (comparative examples) are all inferior in intergranular corrosion resistance.
Further, No. containing no appropriate amount of Ca and (mass% Si × mass% Al) is also less than 1 × 10 ⁻³ . The steel plates 21-24 (comparative examples) are inferior in hot workability.

  Since the duplex stainless steel of the present invention is excellent in intergranular corrosion resistance, it is not limited to materials for urea plants. For example, materials used in other fertilizer plants and high chloride environments It can be suitably used as a corrosion-resistant material used in oil wells.

Claims (5)

C: 0.05 mass% or less, Si: 1 mass% or less, Mn: 0.3 to 2 mass%, P: 0.04 mass% or less, S: 0.002 mass% or less, Al: 0.1 mass% or less, N: 0 0.05 to 0.4 mass%, Ni: 5 to 8 mass%, Cr: 20 to 27 mass%, Mo: 2 to 5 mass%, B: ≦ 0.01 mass%, Ca: more than 0.0005 mass% and 0.01 mass% or less Containing, and
Two phases for urea plant, characterized in that P and B satisfy (mass% P × mass% B) ≦ 12 × 10 ⁻⁵ , and the remainder has a composition composed of Fe and inevitable impurities Stainless steel. The average corrosion rate of the non-sensitized specimen in the intergranular corrosion test using a 65 mass% boiling nitric acid solution defined in ASTM A262 Practice C is 0.2 g / m ² · hr or less. Duplex stainless steel described in 1. 3. The duplex stainless steel according to claim 1, wherein the Si, P, and B satisfy (mass% Si × mass% P × mass% B) ≦ 5 × 10 ⁻⁵ . 4. The duplex stainless steel according to claim 1, wherein the Si and Al satisfy (mass% Si × mass% Al) ≧ 1 × 10 ⁻³ . 5. The duplex stainless steel according to any one of claims 1 to 5, further comprising Mg: 0.0001 to 0.01 mass% in addition to the component composition.

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